It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When the mu'adhdhin says: 'Allahu akbar, Allahu akbar' (Allah is the Greatest, Allah is the Greatest), you too say: 'Allahu akbar, Allahu akbar'. Then he says: 'Ashhadu an la ilaha illallah' (I bear witness that there is no god but Allah), and you say: 'Ashhadu an la ilaha illallah'. Then he says: 'Ashhadu anna Muhammadan rasulullah' (I bear witness that Muhammad is the Messenger of Allah), and you say: 'Ashhadu anna Muhammadan rasulullah'. Then he says: 'Hayya 'ala al-salah' (Come to the prayer), and you say: 'La hawla wa la quwwata illa billah' (There is no might and no power except with Allah). Then he says: 'Hayya 'ala al-falah' (Come to success), and you say: 'La hawla wa la quwwata illa billah'. Then he says: 'Allahu akbar, Allahu akbar', and you too say: 'Allahu akbar, Allahu akbar'. Then he says: 'La ilaha illallah' (There is no god but Allah); and whoever says 'La ilaha illallah' from his heart will enter Paradise."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1926